The classifications given by railroads to their steam locomotive fleets were as varied as the machines themselves with several different designations within a particular class! Illinois Central Railroad steam locomotives were likewise distinguished by a wide range of numerical classes, a bit from the norm when most lines categorized their fleets by letters. In any event, the information here is most certainly not a complete, all-time listing of the IC's steam fleet and also is merely meant to list the general types of steam locomotives operated by the railroad.
The Illinois Central slogan described the railroad quite well, The Main Line of Mid-America. It was one of only a very few railroads to serve markets with north-south running main lines and not the traditional east-west movements. What made its routing even more odd was that it served Midwestern markets that likewise traditionally moved goods east and west, such as Chicago, St. Louis, Memphis, and New Orleans. Regardless of this the IC carved out a living hauling goods from Chicago to New Orleans and while today the Canadian National owns the railroad, its name continues to survive after over 150 years of existence.
The IC operated several different types of steam locomotives including 2-8-4 Berkshires, 2-10-2 Santa Fes, and 2-6-6-2 heavy articulateds. While there are currently none in operation several IC steamers survive, including two 4-8-2 Mountains.
Class 10
The IC's Class 10 included its fleet of 4-6-0 Ten-Wheelers.
Class 65, 75, 85, 102, 200, 210, 250
These classes designated IC's fleet of 0-6-0 switchers.
Class 401, 3700
These classes designated IC's fleet of 2-6-0 Moguls.
Class 541
The IC's Class 541 included its fleet of 2-6-0 Moguls.
Class 640
The IC's Class 640 included its fleet of 4-8-0 Twelve-Wheelers.
Class 700, 701, 790, 900
These classes designated IC's fleet of 2-8-0 Consolidations.
Class 1000, 1031, 1049, 1139, 1310, 2030
These classes designated IC's fleet of 4-6-2 Pacifics.
Class 1001, 1002
These classes designated IC's fleet of 4-4-2 Atlantics.
Class 1427
The IC's Class 1427 included its unique 2-4-6T.
Class 1442, 1449, 1454
These classes designated IC's roster of 2-6-4Ts.
Class 1501, 2100, 2101, 3766
These classes designated IC's roster of 2-8-2 Mikados.
Class 2300, 2350, 2400, 2500, 2600
These classes designated IC's roster of 4-8-2 Mountains.
Class 2499
The IC's Class 2499 was its only roster of 4-6-4 Hudsons.
Class 2800, 2901
These classes designated IC's roster of 2-10-2 Santa Fes.
Class 3400, 3500, 3555
These classes designated IC's roster of 0-8-0 switchers.
Class 3600
The IC's Class 3600 was its only roster of 0-10-0 switchers.
Class 3610
The IC's Class 3610 was its only roster of 2-10-0 Decapods.
Class 3650
The IC's Class 3650 was its only roster of 0-8-2 switchers.
Class 6000
The IC's Class 6000 was its only roster of 2-6-6-2 heavy articulateds.
Class 7000, 7050, 8000
These classes designated IC's roster of 2-8-4 Berkshires.
For more reading and history on the IC consider Illinois Central Railroad from Tom Murray and published by Motor Books International (MBI has published numerous railroad books on a wide range of topics covered, and overall they do a very good job with books that are reasonably priced). The book gives a general history of the IC and includes several photographs, many in color. If you have any kind of interest in the IC you should very much enjoy this publication.
Also consider Perfecting the American Steam Locomotive by author J. Parker Lamb. As the name implies the book details the earliest history of steam engine technology, even before it was used in railroad applications. His book later explores the development of steam locomotive technology in the United States from the 19th through the 20th centuries, covering not only the most popular steam locomotive designs but also the most successful manufactures to build them. The book has received excellent reviews and is a great resource on steam locomotives and a fine reference tool; you should find it very useful. If you're interested in perhaps purchasing either (or both) of these books please visit the links below which will take you to ordering information through Amazon.com, the trusted online shopping network.